diff --git a/codec/decoder/core/src/decode_slice.cpp b/codec/decoder/core/src/decode_slice.cpp
index dca0d5e0..c78cab28 100644
--- a/codec/decoder/core/src/decode_slice.cpp
+++ b/codec/decoder/core/src/decode_slice.cpp
@@ -93,7 +93,11 @@ int32_t WelsTargetSliceConstruction (PWelsDecoderContext pCtx) {
     }
 
     ++iCountNumMb;
-    ++pCtx->iTotalNumMbRec;
+    if (!pCurLayer->pMbCorrectlyDecodedFlag[iNextMbXyIndex]) { //already con-ed, overwrite
+      pCurLayer->pMbCorrectlyDecodedFlag[iNextMbXyIndex] = true;
+      ++pCtx->iTotalNumMbRec;
+    }
+
     if (iCountNumMb >= iTotalNumMb) {
       break;
     }
@@ -394,7 +398,6 @@ int32_t WelsDecodeSlice (PWelsDecoderContext pCtx, bool bFirstSliceInLayer, PNal
     }
 
     ++pSlice->iTotalMbInCurSlice;
-    pCurLayer->pMbCorrectlyDecodedFlag[iNextMbXyIndex] = true;
 
     if (pSliceHeader->pPps->uiNumSliceGroups > 1) {
       iNextMbXyIndex = FmoNextMb (pFmo, iNextMbXyIndex);